Obtain an optical "bench" setup (there should be three sliding mounts on the calibrated horizontal bar. The setup is shown in the diagram below.

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Astronomy 100 Name(s): Exercise 4: Telescopes and spectroscopy Once the various focal issues are resolved, magnification of a small image is a significant consideration for a telescope.

1 Astronomy 100 Name(s): Exercise 4: Telescopes and spectroscopy Once the various focal issues are resolved, magnification of a small image is a significant consideration for a telescope. Though a planet is quite large, its image in the eyepiece is quite small; in order to see detail, the image must be magnified. Consider the angular size of a planet; the disk of Mars is virtually impossible to distinguish with the unaided eye. However, a telescope will magnify the apparent angular size of any object; you will be able to see the disk of Mars. The diagram below, from Hewitt s Conceptual Physics, shows the light rays and lenses of a telescope: The equation for the magnification is simple: M = f objective /f eyepiece where M is the magnification (usually expressed with an "X" at the end, such as "10X"), and the f's are the focal lengths of the objective and eyepiece lenses. Obtain an optical "bench" setup (there should be three sliding mounts on the calibrated horizontal bar. The setup is shown in the diagram below. Find the centimeter markings on the horizontal bar. Place the light source (on its holder) at 0 cm. Place the screen with the arrows at 5 cm. This exercise will probably work better with all room lighting off. The objective lens (AKA the primary lens) should be placed in the spring-loaded holder, and the eyepiece lens (the smaller ones) should be placed in the screw-

2 down holders. You will be switching eyepiece lenses, so practice removing and installing lenses in that holder; don t tighten the screw so much that the lenses crack! Begin by placing the objective lens at the 50 cm marker on the horizontal bar, and removing the eyepiece lens holder entirely. Place the frosted glass viewscreen (with cross-hairs) on the other side of the objective lens from the light source. Make sure that an image of the arrows is visible on the viewscreen. Move the mount with the frosted glass screen along the calibrated bar to obtain a focussed image (upside down and reversed) of the arrows. Do not move the objective lens mount! Determine the focal length of the objective lens, and enter it in the appropriate square of the table below. Remove the objective lens and its holder and replace it with one of the smaller eyepiece lenses (record the letter associated with the lens) in its holder. Mount it at 50 cm. Determine the focal length of the eyepiece lens, and enter it into the table below. Calculate the magnification using the equation above. Replace the objective lens (don t remove the eyepiece lens) at 50 cm. Move the eyepiece lens so that it is at a distance from the objective lens which is exactly the sum of the objective lens focal length and the eyepiece length focal length (in other words, the sum of columns two and three). View the image of the arrows through the lens. 1. The image of the arrows should be in focus; adjust the position of the lens if it is not. Is it right side up or upside down and backwards? Is the arrow image magnified from what it looks like at the same distance without the lenses? 2. Calculate the magnification for various telescopes to compare and contrast magnification abilities. Under type of telescope, state whether the telescope is a refractor or reflector type of telescope. Name of device type of telescope Optical bench telescope Focal length of objective (fobjective) in centimeters Focal length of eyepiece (feyepiece) in centimeters Magnification power Meade 125 mm telescope Meade LX-200 telescope

3 3. a. What general rule(s) can you formulate about the focal length of the objective lens, the focal length of the eyepiece lens and higher magnification? b. Examine the eyepiece lenses from actual telescopes; what do you notice is the drawback to the simple rule of get a smaller focal length eyepiece? Resolution Resolution is simply the angular size of the smallest object you can distinguish in a telescope s field of view. A simple equation to determine the resolution of a telescope is: & λ ) θ 250,000 ( + ' D* where λ is the wavelength of incident light in meters, D is the diameter of the objective lens or mirror of the telescope in meters, and θ is the angular resolution in arcseconds. 4. a. For the telescope you will set up in the last part, assume we are viewing a green light (λ = 550 nm). Measure the diameter of the primary lens and determine the angular resolution of your telescope. b. Will you be able to see individual lunar craters (about 5 arcseconds wide) with this telescope? 5. Give two ways in which you could make the angular resolution better. Hint: one of the ways is impossible using glass lenses.

$In fact, this is the reason that all the big telescopes in the world are reflectors and not refractors.$

4 6. Why don't telescope makers simply fabricate huge objective lenses, instead of trying to make huge polished mirrors? In fact, this is the reason that all the big telescopes in the world are reflectors and not refractors. (Hint: Consider the material lenses are generally made from and the deformation properties of such material) Introduction to spectroscopy How do astronomers know anything about parts of the solar system and universe that we (humans or the probes we send) have never physically touched? The answer lies in the light that is emitted or reflected by an astronomical object. In this exercise, you will experimentally investigate objects that emit light energy and objects that absorb light energy, and how you measure this absorption or emission. To do this, you will need to know the term electromagnetic spectrum which is the whole range of energies (wavelengths, frequencies) that light can have. For this experiment, we will be interested primarily in the visible spectrum, a tiny subset of the electromagnetic spectrum. The device you will use to measure the spectrum is the spectroscope (called the SCSpec in this experiment), which is basically a smart prism. A prism because, like its glass cousin, it separates light into its component colors (ROYGBV), and smart, because it can tell you what wavelengths are present in the spectrum.

5 Gathering the data Work in groups of three or four; the order in which you do questions #1 #6 does not matter. Needed: Laptop computer from the cart (pick from #1 to #7) SCSpec spectroscope kit (in plastic tub) Set up the spectroscope and laptop as stated in the laminated sheet that comes with the spectroscope kit. Make sure that you have a Desktop visible on the laptop before you plug the USB cable from the spectroscope into the laptop. Make sure the spectroscope is actually plugged into an electrical socket. Follow the instructions on the laminated sheet to start up the software SCSpec. After clicking on the Connect button, it should quickly turn into a Disconnect button. If this does not happen, let me know! You should see a rainbow-like set of colors appear on the left of the screen. Do not worry about Calibration ; after setup, go directly to the back of the sheet to Capturing Spectra. You will not need to Save any of the data you collect since you ll be writing it down, but you will need to make sure that Graph is visible. After clicking Graph, you should see a graph appear on the right of the screen. 7. a. Look at the graph and state what quantity the x-axis represents, and what units that quantity is in. Hint: see diagram on first page of this handout. b. What quantity does the y-axis represent, and why might the units not be so helpful for this quantity? 8. Take the laptop and spectroscope outside (please be careful not to drop anything), and use the extension cord to plug in the spectroscope. Point the spectroscope at a piece of white paper that is reflecting sunlight. DO NOT POINT THE SPECTROSCOPE DIRECTLY AT THE SUN; the results are not useful. Sketch the graph below, giving numbers on the x-axis where useful.

6 9. While still outside, point the spectroscope at nearby shrubbery and sketch that graph below, again giving numbers on the x-axis where useful. 10. Indoors, point the spectroscope at the fluorescent light. Sketch the graph below, giving numbers on the x-axis where useful. 11. Indoors, obtain the ultraviolet (UV) box and carefully point the spectroscope up at the UV light. DO NOT LOOK AT THE ULTRAVIOLET LIGHT YOURSELF unless you wish to have cataracts. Sketch the graph below, giving numbers on the x-axis where useful.

7 12. Indoors, point the spectroscope at a low wattage incandescent light bulb. Try to do this in such a way as to minimize the amount of the fluorescent light pointed at the spectroscope. Sketch the graph below, giving numbers on the x- axis where useful. 13. Now point the spectroscope at a high wattage light bulb. Again, try to do this in such a way as to minimize the amount of the fluorescent light pointed at the spectroscope. Again, sketch the graph below, giving numbers on the x-axis where useful. 14. Just for grins, point the spectroscope at the surface of a dark object (like the table) and sketch its graph.

8 15. Find a gas discharge lamp with a hydrogen gas tube. Using the spectroscope pointed at the gas discharge tube with the appropriate gas, measure the wavelength of the emissions (pay attention to the units!). Unlike the previous parts, no graph is necessary but roll the cursor onto the peaks of the graph to get numerical data (look for the x-coordinate in the lower right of the screen). You may need to calibrate the spectroscope. Fill in the table with these data. Find a gas discharge lamp with a helium gas tube, and do the same. Repeat with the oxygen gas tube, and finally, the water vapor tube. Tube gas Colors (lines) visible list both the color and the wavelength (length units?) Hydrogen Helium Oxygen Water vapor 16. I will demonstrate the infrared (IR) camera. What characteristic do all the bright areas of the IR image show? Check this by looking at footprints and ice. 17. Under what circumstances will an IR camera see a star that a visiblewavelength camera can t? Make a schematic sketch of this, showing the star and the IR and visible-wavelength cameras and any other stuff needed to illustrate your point. At this point, put away the laptop and spectroscope; make sure all the parts return to their rightful place. Keep this data for analysis in the next exercise.

For instance, for a particular star cluster, these data were derived:

For instance, for a particular star cluster, these data were derived: Astronomy 100 Name(s): Exercise 5: The H-R diagram and spectroscopy A very basic correlation using the color index By the 1920 s, various astronomers had evidence that the temperature of a star was also